Incandescent lamps exhibiting the so called halogen cycle are well known. Such lamps operate on a tungsten-halogen cycle which is a regenerative, continuous process in which tungsten halide is produced when the halide combines chemically with particles of tungsten evaporating from an incandescent tungsten filament. Subsequent thermal decomposition of this compound replaces the tungsten particles on the filament. This process keeps the tungsten particles from depositing on the lamp envelope and producing a black coating which reduces light output.
For many years the preferred halide employed was iodine. Some years ago it was recognized that the use of bromine in place of iodine had beneficial effects, such as increased efficacy, and this substitution was indeed made, and bromine is now the standard in the industry for short to medium life lamps, i.e., lamps having design lives of 25 to 1500 hours.
However, previous attempts to employ bromine in long life lamps (having design life expectancies from 1500 to 3000 hrs.) have not been successful. By including in the lamp sufficient hydrogen bromide or other forms of bromine to meet the life requirements, it was found that hydrogen would permeate the quartz envelope leaving free bromine to attack the tungsten filament and cause early lamp failure. If an insufficient amount were used, the regenerative cycle would fail and lamp blacking would occur.